Electrical detector of vapors and the like



April 18, 1950 H TE 2,504,772

I ELECTRICAL DETECTOR OF VAPORS AND THE LIKE Filed July 8, 1947 l1 ca5'" RELAY l6 4. i W 3 I m y Ia Inventor:

William C.Whi te,

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y His Attorney- Patented Apr. 18, 1950 ELECTRICAL DETECTOR OF VAPORSLIKE ANDTHE William C. White, Schenectady, N. Y., assignor to GeneralElectric Com New York pany, a corporation of Application Jub 8, 1947,Serial No. 759,641

11 Claims- -(Cl. 175183) This invention relates to improvements in theelectrical methods and apparatus for detecting certain finely dividedatmospheric substances described and claimed in a copending applicationSerial No. 754,657, filed June 14, 1947, by Chester W. Rice, andassigned to the assignee of the present invention. It has for itsgeneral object the provision of a method and apparatus having improvedquantitative sensitivity.

The features of the invention desired to be protected are pointed out inthe appended claims. The invention itself together with its furtherobjects and advantages will be best understood by reference to thefollowing specification taken in connection with the appended drawingwhich represents one embodiment of the invention in a convenientindustrially or otherwise useful vapor detector designed to detect thepresence of finely divided substances and vaporsin any suitable inertatmosphere acting as a, carrier of the substance.

In the aforesaid application of Chester W. Rice, there are described andclaimed methods and apparatus for detecting certain finely dividedsubstances such as certain gases, vapors, smokes or other particles ofmatter, in the atmosphere or elsewhere, by passing a sample of anatmosphere, which may be suspected of containing a concentration of thesubstance to be detected, into an electrical discharge device underconditions which permit the substance to induce positive ion formationat a positively charged heated electrode, the positive ions so formedbeing collected by a negatively charged electrode to produce anindicating current which increases with the concentration of thesubstance. Devices of this type have been found to be qualioccurs byionization of the substance itself when it comes into contact with theheated electrode.

For others, such as the halogens and their compounds, the substanceappears to cause ion formation only in the presence of what may becalled sensitizing materials, such as the alkali metals or theircompounds, and it. appears in this case that the sensitizing materialsthemselves are actually ionized to afford the desired positive ioncurrent.

Now I have found that the quantitative sensitivity (i. e. the rate ofion formation) of devices of this nature may vary with variations in therate of air flow in the space between the electrodes. If that rate istoo rapid, apparently only a limited number of vapor particles haveopportunity to strike the hot positively charged electrode and induceion formation. Therefore, the sensitivity of the device may decrease asthe rate of air flow is increased. On the other hand, while sensitivitymay be higher at low rates, if the air flow has too low a rate, responseof the device to vapor concentration changes will be. slow because ofthe time delay in the passage of the vapor through the device. Moreoverprolonged or excessive exposure of the electrodes to detectable vaporswhich may occur at low air flow rates has a deleterious effect onsensitivity. The speed at which sensitivity recovers from the lattereffect after cessation of the prolonged or excessive exposure is slowbut may be hastened by further fiow of relatively pure air. An optimumrate of air fiow which provides the best compromise between theseconditions may be selected for any given set of operating conditions.

The foregoing indicates the following as one desirable and useful modeof operation of the device when, for example, it is used to detect thepresence of a toxic or other vapor in a room or other space. In orderinitially to take full advantage of the greater sensitivitycharacterizing low air velocities, the device may be permitted initiallyto circulate the air through its interelectrode space only by virtue itsown heat convection currents created by its hot electrodes or by the aidof relatively weak blower action. When an initial concentration of vaporis indicated by an increase in current in the device, the rate of flowmay be increased by suitable means, for example, the increase of anodecurrent may be caused to operate a suitable amplifier and relay, whichmay in turn activate a blower. The blower in turn, will greatly increasethe air flow through the device and thus tend to cause the detector torecover quickly from any decrease in the sensitivity resulting from anyprolonged or excessive exposure occurring because of the initialrelatively higher vapor concentration in the interelectrode space.Moreover, with increased air flow velocity, the time of passage throughthe detector will be small for which reason it will respond more quicklyto changes in the degree of concentration of the substance detected. Ifthe positive ion current subsequently be reduced either because thevapor concentra- Referring now to the drawing it will be noted that anelectronic discharge device I of the type disclosed in theaforementioned Rice application is indicated schematically. It may, forexample, comprise an envelope 2, a heated positively charged electrode3, a negative electrode 4, an air inlet orifice and an outlet orifice G.The heater wire 1 of the electrode 3 may be energized by any suitablemeans such as a full line voltage heater or by the secondary 8 of atransformer 9 suitably connected to the heater wire and in turnenergized by any suitable source of power imposed upon its primary coill0. Any suitable source of voltage for establishing a potentialdifierence between the electrodes 3 and 4 may be employed, for example,the battery II. For the purpose of giving an indication of the magnitudeof the current flowing between the electrodes, there may be provided amicroammeter or simila current detecting instrument l2. The currentrange of microammeter I! will preferably be chosen so as to distinguishbetween any minute currents which might flow between the anode and thecathode in the absence of the substance to be detected and the greatercurrents which will flow when the presence of the substance gives riseto ions creating the detection current.

For the. purpose of causing forced air flow through the device whendesired, any suitable means maybe provided such as a blower l3 having acowling aligned with the outlet orifice 6 and having for example, a fanIt driven by a motor l5, energized by any suitable source of voltage l6.Normally, blower l3 will be either unenergized or energized at arelatively slow speed whereby the rate of air flow through the device Iwill be relatively small for initial detections of vapor. Under thosecircumstances, the sensitivity of the device will be at or near itsmaximum value. After an initial amount of the substance to be detectedhas entered the device, the rate of air flow may be caused to be steppedup in order that the device will respond more quickly to changes in theconcentration of the substance to be detected in the atmosphere and inorder to minimize slow sensitivity recovery and possible loss ofsensitivity due to long continued presence of the substance. As onesuitable means for increasing the air fiow in this manner, I have shownan amplifier and relay I'l connected across and actuated by the voltageof. a resistor l8 in series with the circuit of the electrodes.Amplifier I! may be arranged to energize a solenoid l9 which uponenergization causes a switch 20 to close the energizing circuit for'theblower I3. It will be apparentwith this arrangement when an initialconcentration of vapor is introduced into the devic I by the air streamor by convection currents, the blower circuit will be automaticallyenergized and the air flow increased to produce the aforementionedeifects.

It may be desirable to introduce by well known means a slight time delayin the opening of switch 1 20 in order that the blower may remain inaction long enough to cause the device to be well flushed with air aftera drop in current sufficient to deactivate the relay'. This will removeany residual amounts of the substance which might give rise to currentsinterfering with normal operation of the device and cause erraticaction.

While I have shown and described a particular embodiment of myinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from myinvention in its broader aspects and I, therefore, aim in the appendedclaims to cover all such changes andvmodiflcations as fall within thetrue spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The method ofoperating a detector for a finely divided atmosphericsubstance of the type comprising an electrical discharge deviceincluding a pair of electrodes, means for causing a flow of anatmosphere contaimng said substance to pass between said electrodes,means for imposing an electric potential difference betweensaid'electrodes, means for heating the more positive one of saidelectrodes and means for detecting changes in the current between saidelectrodes caused by the formation of positive ions within said devicedue to the presence of said substance, which method comprises initiallyrestricting said fiow to a relatively slow rate whereby to obtain opti--mum sensitivity of said detector to an initial introduction of saidsubstance thereinto and substantially increasing said rate of fiow afterinitial detection of said substance.

2. The method as in claim 1 in which said substance is of a class havingan ionization potential less than the lectron work function of the morepositive of said electrodes.

3. The method as in claim 1 wherein said substance is of the classconsisting of the alkali metals and compounds thereof.

4. The method as in claim 1 in which said substance is of the classconsisting of the halogen elements and compounds thereof.

5. A detector for a finely divided atmospheric substance comprising anelectrical discharge device including a pair of electrodes, means forcausing a flow of an atmosphere containing said substance to passbetween said electrodes, means for imposing an electric potentialdifference between said electrodes, means for heating the more positiveone of said electrodes and means for detecting changes in the currentbetween said electrodes caused by the formation of positive ions in saiddevice due to the presence of said substance, and means controlling saidfirst mentioned means for initially restricting said fiow to arelatively slow rate. whereby to obtain optimum sensitivity of saiddetector to an initial introduction of said substance thereinto andmeans controlling said first mentioned means for substantiallyincreasing said rate of flow after initial detection of said substance.

6. A detector as in claim 5 in which said substance is of a class havingan ionization potential less than the electron work function of the morepositive of said electrodes.

'7. A detector as in claim 5 wherein said" substance is of the classconsisting of the alkali metals and compounds thereof.

8. A detector as in claim 5 in which said substance is of the classconsisting of the halogen elements and compounds thereof.

9. A detector as in claim 5 wherein said two last mentioned meanscomprise a relay responsive to the fiow of current in said device andadapted to energize said first mentioned means upon initial flow ofcurrent in said device.

causing a flow of atmosphere containing said substances to pass betweensaid electrodes, means for imposing an electrical potential difierencebetween said electrodes, means for heating the more positive of saidelectrodes, and means for detecting the presence of said substance bythe current caused to flow between said electrodes by the formation ofpositive ions within said device and means responsive to the changes incurrent flow in said device adapted to energize said first mentionedmeans for increasing said flow after initial detection of saidsubstance.

11. The method of operatin a detector of finely divided substances in anatmosphere which detector has high sensitivity at low rates of flow ofsaid atmosphere through said detector but is readily subject tocontamination at low rates of 3 initial introduction of said substancethereinto and substantially increasing said rate of flow after initialdetection of said substance.

WILLIAM C. WHITE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Name Date Y Goddard June 9, 1931 Salzberg et alNov. 16. 1943 Number

